1. Field of the Invention
The invention relates to surface acoustic wave (SAW) devices and methods, and specifically to devices and methods for achieving direct generation and detection of a coded minimum shift keyed (MSK) waveform.
2. Background
Surface acoustic wave devices are well known in the art. Typically, a SAW device is characterized by an input interdigital transducer (IDT) arranged on a piezoelectric surface with an output transducer spaced from the input transducer for detecting and processing a surface acoustic wave launched by the input transducer along the piezoelectric surface. A number of electronic signal processing functions have been devised using a variety of circuits incorporating SAW devices. In particular, the use of SAW filters has become a preferred method for implementing continuous phase shift modulation in spread-spectrum communication systems.
U.S. Pat. No. 3,969,590 issued to Anant K. Jain and Robert K. Marston on Jul. 13, 1976 and assigned to the Rockwell International Corporation discloses a modulating system using a SAW device for receiving impulses and providing shaped carrier signals as an output for transmitting data. The apparatus teaches the use of a SAW device as a means of producing specific outputs in the form of a modulated signal and specifically teaches the application of an impulse to the device to produce a MSK modulated output without the use of auxiliary phase lock loops, balanced modulators or other means typically required of such signal modulation devices not incorporating SAW techniques. In the device disclosed, a phase shift keyed (PSK) waveform is digitally generated first and then a SAW filter is used to convert the PSK waveform to an MSK waveform.
U.S. Pat. No. 4,592,009 issued to Michael S. Masheff on May 27, 1986 and assigned to E-Systems, Inc. discloses an MSK SAW convolver for use in a communications system in which a signal input incorporates a mixed pseudo noise (PN) code sequence and narrow band information signal phase modulated thereon according to an MSK modulated scheme. The convolver includes a piezoelectric substrate SAW device having a first transducer formed on the top surface for receiving a signal and generating acoustic surface waves along a first direction. A second transducer is also formed on the top surface for receiving a reference signal and in response generating acoustic surface waves along a second direction. The reference signal incorporates the PN code sequence phase-modulated thereon according to a bi-phase shift keying (BPSK) modulation scheme. A BPSK-MSK converter integrally formed on the substrate .produces the reference signal in MSK form.
U.S. Pat. No. 5,033,063 issued to Takeshi Okamoto and Shoichi Minagawa on Jul. 16, 1991 and assigned to the Clarion Co. Ltd. also discloses a SAW device for use in spread-spectrum communications. The patent teaches a SAW device comprising at least two SAW transducers made from metal and disposed along the surface acoustic wave propagating direction on a piezoelectric substrate. At least one of the transducers has a different bandwidth and a convolver output gate electrode in the form of a rectangular metal layer placed between the transducers. Both this patent and the Masheff U.S. Pat. No. 4,592,009 described above use schemes in which a PSK waveform is generated digitally and then converted to an MSK waveform by a SAW transducer embedded alongside a SAW convolver used to detect the received MSK waveform.
The fact that MSK modulated spread spectrum waveforms consisting of a sequence of contiguous pulses, each of which is a short burst at one of two different frequencies, have the advantage of lower spectral side lobes and thus lower cross-channel interference than do PSK waveforms is well known in the art. As described by W. Richard Smith of the Hughes Aircraft Company, Fullerton, Calif. in a paper presented during a 1977 IEEE Ultrasonic Symposium Proceedings, progress in coded, spread-spectrum communication systems has shown the need to avoid spillage of energy from one communication channel into the frequency bands allocated for adjacent and nearby communication channels. It is pointed out that this is a principal reason for using MSK waveforms, also known as continuous phase shift modulated (CPSM) waveforms, for direct sequence coding in spread spectrum communication systems. Smith discloses an MSK modulator using a SAW filter. In the scheme disclosed, an input coded pulse train is mixed with a carrier wave to produce a PSK waveform. The PSK waveform then becomes the input to the SAW filter to produce an MSK waveform. Smith concludes that the use of SAW filters will undoubtedly become the method of choice for implementing MSK modulation. Their simplicity reduces the size and cost, and by virtue of lacking critical time alignment problems, they have allowed MSK waveforms to be generated at higher carrier frequencies than previously possible.